1. Field of the Invention
The present invention relates to a polyimide film which has a high elastic modulus, a low thermal expansion coefficient, alkali etchability and excellent film-forming properties when used as a metal interconnect board substrate on the surface of which metal interconnects are provided in such applications as flexible printed circuit and tape-automated bonding (TAB) tape. The invention relates also to a method for manufacturing such film. The invention additionally relates to a metal interconnect board for use in flexible printed circuits or TAB tape in which the foregoing polyimide film serves as the substrate.
2. Description of the Related Art
TAB tape is constructed of a heat-resistant film substrate on the surface of which are provided very fine metal interconnects. In addition, the substrate has openings, or “windows,” for mounting integrated circuit (IC) chips. Sprocket holes for precisely feeding the TAB tape are provided near both edges of the tape.
IC chips are embedded in the windows on the TAB tape and bonded to the metal interconnects on the tape surface, following which the mounted chip-bearing TAB tape is bonded to a printed circuit for wiring electronic equipment. TAB tape is used in this way to automate and simplify the process of mounting IC chips on an electronic circuit, and also to improve manufacturing productivity and enhance the electrical characteristics of electronic equipment containing mounted IC chips.
TAB tapes currently in use have either a three-layer construction composed of a heat-resistant substrate film on the surface of which an electrically conductive metal foil has been laminated with an intervening layer of polyester, acrylic, epoxy or polyimide-based adhesive, or a two-layer construction composed of a heat-resistant substrate film on the surface of which a conductive metal layer has been directly laminated without an intervening layer of adhesive.
The substrate film in TAB tape is thus required to be heat resistant. Polyimide film in particular has been used to ensure that the substrate film is able to withstand high-temperature operations such as soldering when IC chips are bonded to the metal interconnects on TAB tape and when the IC chip-bearing TAB tape is bonded to a printed circuit for wiring electronic equipment.
However, the heat incurred in the process of laminating polyimide film with metal foil or a metal layer then chemically etching the metal foil or metal layer to form metal interconnects may elicit differing degrees of dimensional change in the polyimide film and metal, sometimes causing considerable deformation of the TAB tape. Such deformation can greatly hinder or even render impossible subsequent operations in which IC chips are mounted on the tape and the IC chip-bearing TAB tape is bonded to a printed circuit for wiring electronic equipment. Accordingly, a need has been felt for some way to make the thermal expansion coefficient of polyimide film closer to that of the metal so as to reduce deformation of the TAB tape.
Moreover, reducing dimensional change due to tensile and compressive forces in TAB tape on which IC chips have been mounted and which has been bonded to a printed circuit for wiring electronic equipment is important for achieving finer-pitch metal interconnects, reducing strain on the metal interconnects and reducing strain on the mounted IC chips. To achieve this end, the polyimide film used as the substrate must have a higher elastic modulus.
According to the definition of a polymer alloy or blend (see “Polymer Alloys: New Prospects and Practical Applications,” in High Added Value of Polymer Series, edited by M. Akiyama and J. Izawa, published in Japan by CMC K. K., April 1997), block copolymerization, blending, interpenetrating polymer network (IPN) formation and graft polymerization all fall within the category of processes capable of increasing the elastic modulus of a polymer.
With respect to polyimides in particular, Mita et al. (J. Polym. Sci. Part C: Polym. Lett. 26, No. 5, 215-223) suggest that, on account of the molecular composite effect, a blend of different polyimides can more readily attain a high elastic modulus than a copolyimide obtained from the same starting materials. However, because polyimide molecules have large molecular cohesive forces, mere blends of such molecules tend to take on a phase-separated structure. Some form of physical bonding is needed to inhibit such phase separation.
An interpenetrating network polymer was proposed for this very purpose by Yui et al. (“Functional Supermolecules: Their Design of and Future Prospects,” in New Materials Series, edited by N. Ogata, M. Terano and N. Yui, published in Japan by CMC K. K., June 1998).
A specific example of a blend according to the prior-art is disclosed in JP-A 63-175025, which relates to polyamic acid compositions (C) made up of a polyamic acid (A) of pyromellitic acid and 4,4′-diaminodiphenyl ether and a polyamic acid (B) of pyromellitic acid and phenylenediamine. This prior art also discloses polyimides prepared from such polyamic acid compositions (C).
However, the methods provided in this prior art involve first polymerizing the different polyamic acids, then blending them together. Because thorough physical interlocking of the type seen in an interpenetrating network polymer cannot be achieved in this way, phase separation may occur during imidization of the polyamic acid. In some cases, a slightly hazy polyimide film is all that can be obtained.
JP-A 1-131241, JP-A 1-131242, U.S. Pat. No. 5,081,229 and JP-A 3-46292 disclose block copolyimide films manufactured from block copolyamic acids composed of pyromellitic dianhydride, p-phenylenediamine, and 4,4′-diaminodiphenyl ether. This prior art also discloses methods for manufacturing copolyamic acid films composed of block components of ultimately equimolar composition by reacting non-equal parts of the diamines and the acid dianhydride in an intermediate step.
However, in such prior-art processes, although the polyamic acid blend solution prepared is not prone to phase separation, the molecular composite effect is inadequate and a satisfactory increase in rigidity is not always achieved. Moreover, because polymer production involves copolymerization using block components in which the molecular chains are regulated, the reaction steps are complex and reaction takes a longer time. Also, the reaction passes through a step in which there exists an excess of reactive end groups, which tends to destabilize the polyamic acid in the course of production, making it subject to changes in viscosity and gelation. In addition to these and other production problems, the above prior-art methods sometimes fail to provide a film having a sufficiently high Young's modulus.
The surface of the polyimide film substrate is sometimes roughened by etching with an alkali solution prior to use so as to improve the adhesive strength of an adhesive applied thereto. Alkali etching is also at times used to form through holes or vias for interconnects. Accordingly, there has arisen a desire for polyimide films having excellent alkali etchability.
A film having good planarity is desirable for better ease of handling in processing operations. The planarity of the film can be improved by increasing the stretch ratio during film production. Hence, film compositions capable of being subjected to orientation at a high stretch ratio are also desired.
Methods for producing polyimide films which satisfy such requirements have already been proposed. For example, JP-A 1-131241, JP-A 1-131242 and JP-A 3-46292 provide polyimide films made from polyamic acid prepared from pyromellitic dianhydride, p-phenylenediamine, and 4,4′-diaminodiphenyl ether. The same prior art also teaches processes for producing block component-containing polyamic acid film by reacting non-equal parts of diamine and acid dianhydride in an intermediate step.
However, the above-described prior art methods provide polyimide films which have properties when used as a substrate for metal interconnect boards, which need to be improved.
It is therefore an object of the invention to provide a polyimide film which has a high elastic modulus, a low thermal expansion coefficient, alkali etchability and excellent film formability when used as a metal interconnect board substrate of a type that can be provided on the surface with metal interconnects to form a flexible printed circuit, chip scale packages, ball grid arrays or TAB tape. Another object of the invention is to provide a method of manufacturing such a film. A further object of the invention is to provide a metal interconnect board in which the foregoing polyimide film serves as the substrate.